Infrared (IR) sensors of a proposed type, and integrated-circuit imaging devices containing planar arrays of such sensors, would be based on a combination of (1) temperature-dependent ferromagnetism and (2) the use of giant magnetoresistance (GMR) to detect changes in ferromagnetism. Unlike many infrared sensors, the proposed sensors would not have to be cooled to very low temperatures; indeed, they could be designed for optimum performance at room temperature.

A sensor according to the proposal (see figure) would include a layer of ferromagnetic material coated with an infrared-absorbing material. The ferromagnetic material would be heated by incident infrared radiation. The chosen ferromagnetic material would be one with a Curie temperature (Tc) somewhat above the intended operating temperature; for example, MnAs (Tc= 318 K) would probably be suitable for room temperature (≈293 K). The reason for this choice is that the magnetization of a ferromagnetic material decreases sharply with increasing temperature as the temperature approaches Tc, so that the magnetic field produced by such a material can serve as a sensitive indicator of temperature in the range just below Tc. Hence, one would expect the magnetic field of the ferromagnetic material to decrease when the material was heated by incident infrared radiation.

Infrared Radiation would be absorbed, causing warming of the ferromagnetic layer to near its Curie temperature and thus a decrease in magnetization. The GMR portion of the sensor would sense the change in magnetization.

A GMR sensor is a multilayer device (for example, see the preceding article) that exhibits a change in electrical resistance when exposed to a change in a magnetic field. Some GMR sensors are capable of detecting changes in magnetic-flux density as small as 1 milligauss (10-7 Tesla). The GMR portion of the proposed sensor would sense the temperature-dependent change in the magnetic field of the ferromagnetic layer. The ferromagnetic layer would be separated from the GMR layers by a nonmagnetic layer with a thickness chosen to provide both magnetic coupling and sufficient thermal insulation to enable the temperature of the ferromagnetic layer to rise appreciably upon exposure to infrared radiation.

One of the advantages of both ferromagnetic materials and GMR sensors is that they can be engineered to operate at a chosen temperature with acceptably high sensitivity. Furthermore the layers of a GMR sensor are easily grown by sputter deposition and molecular-beam epitaxy. According to the proposal, the GMR layers would be deposited directly on a readout integrated circuit, followed by deposition of the nonmagnetic, ferromagnetic, and infrared-absorbing layers.

This work was done by Sir B. Rafol of Caltech for NASA's Jet Propulsion Laboratory. NPO-20929

Photonics Tech Briefs Magazine

This article first appeared in the March, 2001 issue of Photonics Tech Briefs Magazine.

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